Location and Surface Species of Fluid Catalytic Cracking Catalyst

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Location and Surface Species of FCC Catalyst Contaminants: Implications for Alleviating Catalyst Deactivation Ubong Jerome Etim, Pingping Wu, Peng Bai, Wei Xing, Rooh Ullah, Fazle Subhan, and Zifeng Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02505 • Publication Date (Web): 25 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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Location and Surface Species of FCC Catalyst Contaminants: Implications for Alleviating Catalyst Deactivation U. J. Etim, † Pingping Wu, † Peng Bai, † ,* Wei Xing, † Rooh Ullah, † Fazle Subhan, †,‖ Zifeng Yan† ,* †

State Key Laboratory of Heavy Oil Processing, PetroChina Key Laboratory of Catalysis,

College of Chemical Engineering, China University of Petroleum, Qingdao, 266555, China ‖

Department of Chemistry, Abdul Wali Khan University Mardan, K.P, Pakistan

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

* Corresponding authors. Tel: +86-532-86981856; 86981296; Fax: +86 532 86981295. E-mail address:[email protected] (P. Bai), [email protected] (Z. Yan)

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ABSTRACT The deposition of appreciable amounts of metal poisons and carbon poses serious problems to the refiner during fluid catalytic cracking (FCC) unit operation. To check the effects of these contaminants on the catalyst, an in-depth understanding of their locations and existing states becomes necessary. In this work, the location and nature of vanadium, nickel and coke species on FCC catalyst were investigated. Detailed analyses of catalyst samples, including industrial equilibrium catalysts (E-cats), were accomplished by using a variety of characterization techniques. It was found that nickel and vanadium concentrated mainly in mesopores and micropores of the FCC catalyst, respectively. On the surface of E-cats, vanadium exists mainly in +4 and +5 oxidation states, while nickel is present as NiO, NiAl2O4 and surface nickel hydrosilicates, and as NiO and NiAl2O4 in the bulk. The formation of a large amount of NiAl2O4 on alumina support by nickel indicates its preferential location in the alumina component of the FCC catalyst. When co-existing, a synergic effect between vanadium and nickel is likely. On the other hand, coke distributed within the catalyst pore spaces, exhibiting different behaviors in different catalysts, due to the effects of the metals and steam treatment. The coke deposits consist of a layer of graphitized carbon with both hydrocarbon and aromatic carbon species. Results obtained in this study provide insights into the nature of contaminants of FCC catalysts and could help in the rational design of catalysts to alleviate the metal poisons during catalytic cracking.

Keywords: FCC catalyst, Metal poisons, Coke, Species, Location

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1. INTRODUCTION Studies on the contamination of the fluid catalytic cracking (FCC) catalyst by metal poisons are an integral part of the overall assessment of the catalyst performance for utilization in the industrial units. As is well recognized, the deactivation of the cracking catalyst in the industrial unit still remains one of the gravest problems plaguing the petroleum refinery operating the FCC unit. As the abundance of sweet crude reserves constantly decreases and heavy cuts becoming more available and affordable, high amounts of organometallic complexes present in the latter class of feeds are problematic to the FCC catalyst. During FCC unit operation, feedstocks, most especially residues, containing inherent metallic elements including vanadium, nickel, calcium, sodium and iron are fed to the riser. Upon contacting with the cracking catalyst, these metals deposit on the catalyst surface. During regeneration, certain transformations, such as the change in valence states and migration occur, which lead to the increased significance of side reactions and the deterioration of cracking products. Besides, the destruction of important cracking characteristics of the catalysts is observed.1-3 The catalysts can retain its activity and other cracking properties if certain measures are taken for feedstocks containing high concentrations of metal poisons. The development of approaches to controlling these problems requires the knowledge of what species the metals are, and where they deposit in/on catalyst in the first place. In addition to metals, the deposition of coke covers the active surface of the cracking catalyst and prevents the access of oil molecules to the cracking sites. Although coke can be burned off during regeneration, its initial deposition is considered a serious problem. Previous studies have revealed that nickel deposits on the exterior of the catalyst particle, and slowly diffuses towards the inner part as the catalyst ages.4-7 Petti et al.5 studied the state of 3


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nickel in a commercial FCC equilibrium catalyst (E-cat) using X-ray photoelectron spectroscopy (XPS). It was found that nickel may exist as either nickel aluminate or nickel silicate with a small percentage as NiO, while the particular aluminate phase of nickel in the catalyst was not determined. By using various analytical techniques, it was confirmed that the bulk NiO was absent in E-cats from Italian refineries when the Ni content was less than 10,000 ppmw.8 In the same study, it was also revealed that Ni preferentially located in the alumina particles of the FCC catalysts. In contrast to nickel, vanadium uniformly distributes throughout the entire catalyst particle.9-10 Under FCC unit conditions, vanadium is thought to exist mainly in +5 state, and may exist in other states depending on the section (environment) of the unit or the regenerator operating mode.11 Several studies12-15 have reported the existence of different vanadium species on FCC catalyst. Vanadium in a +5 state was reduced to +4 and even lower oxidation states upon reduction at high temperatures.15 However, these results were collected on a modelled equilibrium FCC catalyst, which may be different from the industrial E-cats under real FCC operating conditions. Besides the deactivation of the cracking catalyst by metals, coke coverage of the catalyst’s surface limits the accessibility of hydrocarbon molecules to the active sites by the pore blockage mechanism.16-17 The surface coverage is probably the first step in the deactivation of the FCC catalyst by coking. Occelli et al.,18 using nitrogen porosimetry, found that coke deposits distributed within the micropores of the FCC catalyst and that the micropore size diminished on coke deposition. Coke settles in zeolite sites due to slow cracking reactions. As coking increases, a portion of the pores (micro/mesopores) of the catalyst particles are partially or completely blocked, reducing the available pore volume for reactant diffusion. The extent of pore obstruction correlates well with the loss of specific surface area and pore volume.17 Moreover, 4


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the effects of coke deactivation on FCC catalyst depend very much on the nature, structure and morphology of coke on catalyst surface.19 In another study, Roncolatto et al.20 reported that coke on the spent industrial FCC catalyst located on the surface of the catalyst microsphere. It is worth to note that the catalyst microsphere consists of a mixture of zeolite and matrix particles, the latter which probably forms the outermost surface layer of the catalyst. As previously mentioned, the deposition of vanadium and nickel presents serious concerns to the refiners, and must be checked in order to achieve the desired economy of the FCC unit. The development of approaches to controlling the problems requires the knowledge of what species the metals are, and where they reside in FCC catalyst. In this study, we aimed to investigate the species and residence of vanadium and nickel on the surface and inside of spent commercial FCC catalysts from a refinery in Shandong, China, using XPS, diffuse UV-vis spectroscopy, and H2-temperature programmed reduction (H2-TPR) techniques. Laboratory modeled E-cats were prepared to enable the determination of the location of nickel and vanadium by nitrogen adsorption and H2-TPR. The nature and species of coke and its distribution within the active and non-active components of the metal poisoned FCC catalyst particle were also investigated. The results obtained (e.g., H2-TPR) indicate that alumina is preferable to be matrix compared with silica for the control of nickel in heavy oil feedstocks because nickel is trapped as inactive NiAl2O4 in the catalyst. It thus follows that this study could help in the choice of appropriate matrix for FCC catalyst, having high metal poisons resistance.

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2. EXPERIMENTAL 2.1 Materials Preparation. A fresh catalyst sample was supplied by HuiCheng Catalyst Company, Qingdao, China. Two Ecat samples were obtained from a refinery in Shandong, China. The fresh catalyst sample denoted as F-cat was impregnated with vanadium and nickel precursor solutions, which include nickel nitrate, ammonium metavanadate, nickel naphthenate and vanadium naphthenate. The obtained samples were denoted as “x wt% V” and “x wt% Ni”, respectively, where “x” represents the weight content of V and Ni. On the other hand, the precursor name was included in the bracket of the sample name, for instance, x wt%V-(N), where -(N) represents naphthenate and, -(V) and -(Nt) represent vanadate and nitrate precursors, respectively. Similarly, metal supported catalysts were prepared by impregnation with nickel and vanadium naphthenates. All components (REY, SiO2 and Al2O3) were impregnated to 1.0 wt% Ni and 0.41 wt% V. The impregnated catalysts were steam treated on a hydrothermal treatment unit for the simulation of E-cats in the laboratory at 760 oC for 5 h with 1.0 L/min water injection rate. The industrial Ecats were calcined in air at 630 oC for 3 h to remove the residual coke. After cracking reactions on a Microactivity (MAT) unit, the laboratory prepared E-cats were regenerated at 630 oC for 3 h in the presence of air. For coke analysis, coke on the catalysts was extracted according to the procedures reported previously.21-23 The reduction of samples was carried out in a high temperature tubular reactor. Nickel impregnated samples were reduced under a pure hydrogen with a flow rate of 50 mL/min at 500 oC for 5 h. Vanadium impregnated samples and industrial E-cats were reduced under similar conditions as for nickel samples except that the temperature used was 700 oC. After reduction, the samples were cooled down to room temperature in a flow of nitrogen, and were immediately transferred into air tight vials. 6


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2.2 Characterization. Powder X-ray diffraction (XRD) was examined on an X'Pert PRO MPD diffractometer at 40 mA and 40 kV using Cu Kα radiation at a speed of 10 °/min from 5° to 70°. Bulk metal concentrations in the samples were measured using PAN Analytical Axios X-ray fluorescence (XRF) spectrometer. XPS data were collected on a PHI-5000 Versa probe with an Al-Kα X-rays source (1486.6 eV), operating at 15 kV and 15 mA. The binding energy of C1s peak at 284.8 eV was considered as the reference to the measured binding energies. H2-TPR was performed on a Quantachrome ChemBET-3000. About 0.1 g of sample was outgassed in an Ar flow (80 mL/min) at 200 °C for 2 h. After cooling down to 50 °C, the sample was then reduced in a gas flow of 10 vol% H2/90 vol% Ar with the temperature increasing from 50 °C to 1000 °C at a heating rate of 10 °C/min. N2 adsorption-desorption isotherms were collected on a Micromeritics Tristar 3000 surface area and porosity analyzer. Before analysis, the samples were evacuated at 300 °C for 3 h. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method, while pore volume was obtained by Barrett-Joyner-Halenda (BJH) method. Fourier transform infrared (FTIR) spectra of the carbon samples were recorded with average of 64 scans at 4 cm−1 resolution on a Nicolet 6700 spectrometer using a KBr disc. Diffuse reflectance spectrophotometer (DRS) was employed to measure the ultraviolet–visible (UV–vis) spectra in the range of 240–800 nm. Raman spectra were recorded on a DXR Raman spectrophotometer (Thermo Fisher Scientific, USA) at room temperature with a 532 nm excitation. 13C cross polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) was measured for the extracted coke and coked catalyst on a Bruker Advance III-400 MHz spectrometer operating at 50 MHz with a spinning rate of 12 kHz using a 4 mm probe head, and a single pulse length of 6 µs ( π/20 °). 7


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2.3 Cracking Reaction. Cracking reactions were carried out on a MAT unit, using vacuum gas oil (VGO) feedstock with the properties listed in our previous report.24 The reaction was conducted using a catalyst-to-oil ratio of 3.0 g/g at a reactor temperature of 500 ± 2 oC. In a typical run, after purging the catalyst loaded reactor with a 30 ml min-1 nitrogen stream for 30 min, 1.140 ± 0.005 g of feedstock was introduced into the reactor through a syringe pump within 75 s, followed by the nitrogen purging at the same flow rate for 15 min. Coke deposited on the catalyst was analyzed by an elemental analyzer (Elementar Vario EL III). 3000

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E-cat 2 E-cat 1

2000

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Figure 1. XRD patterns of E-cats.

Table 1. Properties of Catalyst Samples Properties SABET (m2/g)b Zeolite SA (m2/g) Pore volume (cm3/g) Metal composition (%) Al2O3 SiO2

F-cat 177.8 104.7 0.21

1.2 wt% V-(V)-cat 17.7 3.4 0.08

E-cat-1 70.3 38.0 0.14

E-cat-2 108.6 66.4 0.15

54.12 40.32

53.46 38.52

49.47 39.12

50.04 41.48 8


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V2O5 NiO Na2O CeO2 La2O3 Fe2O3 P2O5 Sb2O3 CaO

2.66 (1.2)a 2.13 0.40 0.78 0.54 0.22 1.21 0.49 0.33 0.09 0.08 0.16 1.46 2.41 1.93 3.53 1.11 0.59 0.65 0.80 0.66 0.07 0.07 0.23 0.84 0.21 0.24 0.30 0.29 0.52 0.96 a b The number in bracket is the nominal V content; SA: Surface area.

3. RESULTS AND DISCUSSION 3.1 Physical Properties and Chemical Composition of Catalyst Samples. Figure 1 shows XRD patterns of spent catalysts obtained from a petroleum refinery in Shandong, China. According to the patterns, it is apparent that E-cat-1 has much weaker reflections of some typical zeolite Y peaks at 2(°) ≈ 6.3, 15.8 and 23.9, attributed to hkl (111), (331) and (533) miller indices, respectively. This indicates that E-cat-1 is more extensively deactivated compared with E-cat-2 due to the serious destruction of zeolite crystalline framework. The peaks position in the XRD patterns shifts slightly to higher angles, due to unit cell size reduction and increase in Si/Al ratio.25 The surface area and chemical composition analyses further reveal the deactivation extent of the samples, which is believed to be driven by vanadium destruction (see Table 1). The composition of metallic elements among the samples varies due to the differences in the age of E-cat samples and the amount of vanadium and nickel incorporated into the F-cat sample. 3.2 Surface Species and Location of Ni and V in FCC Catalyst. XPS spectra in Figure 2 show the elemental composition on the surface of a typical FCC catalyst. In addition to the ‘T’ elements (Si and Al), vanadium, nickel and iron (contaminant metals), and carbon are also detected. According to the XRF results listed in Table 1, E-cat-1 and 1.2 wt% V9


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(V)-cat have different vanadium contents although reasonably close to each other. The difference in the distance from V 2p3/2 binding energies (BE) to the O1s peaks of these two samples is 0.55 eV, suggesting possibly different degrees of interaction between oxygen and vanadium or different distribution of the V-O groups on the surface of the catalysts. The 1.2 wt% V-(V)-cat sample which was steamed treated recorded the highest O1s BE (533.12 eV), but close to that of E-cat-1 (532.91 eV), implying the contamination of the surface oxygen with steam (H2O).26 The presence of lanthanum indicates the catalyst samples, including the E-cats, obtained from the refinery contain REY-type zeolite. The C1s signal ca. 284 eV for all the samples is indicative of the presence of carbon impurities (CO2) adsorbed from the atmosphere after preparation.27 For all samples, the C1s binding energy varies in the range of only 0.03 eV, indicating the formation of the same carbon species. 500000

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(e) V2p3 (d)

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Figure 2. XPS spectra of FCC catalysts: (a) E-cat-1 (b) E-cat-2 (c) 1.2 wt% V-(N) (d) 2.0 wt% Ni-(N) (e) 2.6 wt% V-(V) (f) 3.0 wt% Ni-(Nt). The valence states of vanadium and nickel on the surface of the catalysts were studied by monitoring the energy levels of 2p core electrons (V 2p3/2 and Ni 2p3/2) as shown in Figures 3 10


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and 4. For E-cats and nickel impregnated samples, the BE at 853.8 eV is assigned to NiO and that at ca. 865 eV is a characteristic of nickel compounds formed by strong interaction with alumina and silica (NiTxOy) (Figure 3).5, 28-29 Table 2 shows the species of V and Ni obtained by deconvoluting the V 2p3/2 and Ni 2p3/2 peaks (Figures 3 and 4) of E-cats and metal impregnated samples. Before reduction (Figure 3(I)), E-cat-1 exhibited the presence of both NiO and a nickel compound (NiTxOy). The NiO species was reduced to metallic nickel (Ni0) upon reduction.29 Unreduced Ni impregnated samples showed only NiTxOy, but after reduction little amount of NiO was formed. This may be because some little portions of surface NiTxOy reduced to NiO 5, or the amount of surface NiO was below detectable limit in the unreduced samples. Similarly, for the vanadium containing catalysts, the peak at BE of ca. 516.0 belongs to V4+, and those in the range of 517- 520.0 eV are attributed to V5+ (Figure 4(I))30 because the BE of V 2p electron level increases with the oxidation state of the vanadium ion26. Upon reduction (Figure 4(II)), some of the V5+ species in the samples reduced to V4+. Evidently, for E-cat-1, the V4+/V5+ ratio increased by about 2 folds upon reduction (Table 2).

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(b)

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Figure 3. Ni 2p3/2 XPS spectra of nickel containing catalysts: (I) Non reduced samples; (a) E-cat-1 (b) E-cat-2 (c) 2.0 wt% Ni-(N) (d) 3.0 wt% Ni-(Nt). (II) Reduced samples; (a) E-cat1 (b) E-cat-2 (c) 2.0 wt% Ni-(N) (d) 3.0 wt% Ni-(Nt).

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Intensity (count/s)

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(a)

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Figure 4. V 2p3/2 XPS spectra of vanadium containing catalysts: (I) Non reduced samples; (a) E-cat-1 (b) E-cat-2 (c) 1.2 wt% V-(N) (d) 2.6 wt% V-(V). (II) Reduced samples; (a) Ecat-1 (b) E-cat-2 (c) 1.2 wt% V-(N) (d) 2.6 wt% V-(V).

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Table 2. Species of Vanadium and Nickel in Catalysts Sample (NR) a

Deconvoluted 2p3/2 BE 516.1 517.0 518.0 517.0 519.4 517.7 518.9 519.9 517.5 518.6 519.7 853.8 857.3

Oxidation state V4+ V5+ V5+ V5+ V5+ V5+ V5+ V5+ V5+ V5+ V5+ NiO NiTxOy c

Sample (R) b

E-cat 2

855.2

Ni(OH)2

E-cat 2

2.0 wt% Ni-(N)

857.0 859.7 864.5 857.0 863.5

NiTxOy Satellite NiTxOy Satellite

2.0 wt% Ni-(N)

E-cat 1 V4+:V5+ = 0.64 E-cat 2 1.2 w% V-(N)

2.6 wt% V-(V) E-cat 1

3.0 wt% Ni-(Nt) a

E-cat 1 V4+:V5+ = 1.26 E-cat 2 1.2 w% V-(N)

2.6 wt% V-(V) E-cat 1

3.0 wt% Ni-(Nt)

Deconvoluted 2p3/2 BE 516.1 517.0 518.0 516.7 518.5 516.9 518.0 519.1 516.3 517.5 518.8 852.8 856.4 864.5 853.4 857.1 864.5 853.3 857.0 864.3 853.2 856.5 862.9

Oxidation state V4+ V5+ V5+ V4+/V5+ V5+ V4+/V5+ V5+ V5+ V4+ V5+ V5+ Ni0 NiTxOy Satellite NiO NiTxOy Satellite NiO NiTxOy Satellite NiO NiTxOy Satellite

NR = not-reduced; b R = reduced; c T for Si or Al.

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202 208 215 (a) (b)

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480 766

(b1)

(c) 211

418 481 589 634

(c1)

770

273 200

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Figure 5. DRS UV-vis spectra of selected catalyst samples showing different species of metallic elements in FCC catalyst; (a) E-cat-1, (b) 1.2 wt% V cat, (b1) reduced 1.2 wt% V cat, (c) 2.0 wt% Ni cat, and (c1) reduced 2.0 wt% Ni cat. To gain insights into the transitional states of metal species and molecular interaction between metals and catalyst particles, the UV-vis spectroscopy analysis was carried out and the spectra obtained are presented in Figure 5. All samples exhibit a strong absorption band at 273 nm, corresponding to AlO4 and SiO4 tetrahedrons in FCC catalysts. For the Ni impregnated sample, the absorption band at 211 nm is associated with NiO species that are highly dispersed on the FCC catalyst. Other characteristic but weak bands at 418, 481, 589, 634 and 770 nm are also observed, which are due to Ni species that have interaction with other elements in the catalyst. Busca et al.8 observed a doublet at 589 and 625 nm on the UV-vis spectrum of a Ni/Al2O3 catalyst and assigned this doublet to the d-d transition of NiO in NiAl2O4 species. Therefore, the remaining small and broad peaks at 589 and 634 nm after reduction indicate retention of NiAl2O4 species. In addition, the peaks at 418 and 481 nm suggest the presence of silicates of nickel such as nickel hydroxysilicate.31 For the vanadium impregnated sample, similar to that of the E-cat, 15


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the spectrum exhibits the existence of V5+ and V4+ species, corresponding to the bands at 480 and 766 nm, respectively. These two bands result from V-O bonds of different lengths due to the charge transfer and the d-d transitions. The charge transfer of oxygen to vanadium (O→Vn+) and the d-d transition of Vn+ are dependent on the oxidation state and the local coordination environment.32 As is reported, the V5+ species originates from polymeric V2O5 formed as a result of the hydrothermal environment during FCC catalyst regeneration.33 In the lower frequency regions, for all vanadium catalysts, there exist multiplets of bands, which are attributed to the charge transfer of VOx monomeric species. A similar behavior of vanadium on HUSY was previously reported by Oliveira et al.34 Noteworthy, characteristics of the reduced samples further justify the results of the non-reduced ones. As shown in Figure 5(b1), after reduction, a broad band, which is formed by the 480 nm peak shift in Figure 5(b) and centers at ca. 580 nm, indicates the charge transfer and d-d transition of vanadium species in oxidation states lower than +5. In addition, H2-TPR was utilized to further study the reduction behavior of catalysts. As shown in Figure 6, E-cat-2 shows a nearly similar profile to that of the fresh sample, implying it is less deactivated compared to E-cat-1, in agreement with the XRD data in Figure 1. However, the vanadium impregnated sample resembles E-cat-1 in the TPR profile, which is also similar to that of a catalyst impregnated with both vanadium and nickel as reported previously.35-36 The prominent hydrogen consumption peak at 535 oC is attributed to the reduction of surface monomeric vanadium species.35, 37-38 This behavior is evidenced in Figure 6(II), where the peaks attributed to the reduction of V2O5 and NiO are completely lost or shifted to lower temperatures due to weaker interaction of the remaining oxides in the reduced samples. The high temperature peaks at about 810 and 831 oC for vanadium and nickel containing samples suggest the 16


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interaction of these elements with alumina, forming aluminum vanadate (AlVO4) and nickel aluminate (NiAl2O4), respectively. The formation of nickel aluminate due to the interaction of nickel with alumina is widely recognized, while the formation of aluminum vanadate is under debate.39 Some researchers hold that AlVO4 forms as a solid solution, which is transient during the lifetime of FCC catalyst in the regenerator and is responsible for hydrolysis of Al atoms, leading to vanadium deactivation of the catalyst.40-41 By using V51 MAS NMR technique, we detected the formation of orthovanadate (VO43-) in our previous study.35 Also, Kanervo et al.42 observed two peaks on the TPR profile of alumina supported vanadium catalyst (> 5 wt% V) at greater than 800 oC. Moreover, in the same study, the particular sample that exhibited this behavior showed the presence of crystalline AlVO4 in the Raman spectrum. It was also confirmed by XRD that the second reduction peak was due to the reduction of AlVO4. These evidences and the above findings in this study are convincing enough to suggest the interaction of vanadium with alumina/aluminosilicate materials in FCC catalyst. For this reason, the high temperature reduction (810-816 oC) peak in the TPR profile of vanadium poisoned and steamed FCC catalyst can then be assigned to the formation of vanadium compounds resulting from this interaction. It generally believed that vanadium preferentially attacks the active particle (zeolite) in FCC catalyst.39,43 To investigate this hypothesis in detail, the matrices and zeolite in FCC catalysts were separately contacted with the contaminant metals to study their possible interactions, which reveal the preferential location of the contaminant metals in the FCC catalyst. Figure 7 shows the H2-TPR profiles of metals (V and Ni) on different FCC catalyst components. As indicated, V on both REY and Al2O3 shows two reduction peaks. For V/REY, the reduction of vanadium occurs at ca. 594 oC, a behavior that was previously observed by Oliveira et al.34 On 17


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the other hand, V/Al2O3 presents two reduction peaks, corresponding to the reduction of monomeric V2O5 and polymeric V6O13, respectively. Vanadium behavior on SiO2 was rather difficult to describe as no well-defined peaks could be discerned. However, the weak dual peaks at ca. 525 and 618 oC are related to the reduction of V2O5 particles. Compared with vanadium (V/Al2O3), nickel (Ni/Al2O3) exhibited a higher interaction with alumina, indicated by the large reduction peak at 818 oC, which is a characteristic reduction peak of bulk NiAl2O4 spinel.35, 38, 44 This nickel spinel species is inactive to many catalytic reactions including FCC. The low temperature peak is attributed to the NiO reduction. A broad reduction peak at ca. 707 oC with a shoulder at 795 oC can be observed for Ni/REY catalyst, suggesting the reduction of NiO that have strong interaction with the zeolite. Maia et al. also considered this reduction peak to be attributed to nickel alumino-silicate compound and nickel oligomeric species in zeolite channels, respectively.45 For Ni/SiO2, a large reduction peak at 460 oC is observed, indicating nickel weakly interacts with SiO2. As previously reported,46 Ni forms large particles that are reduced at lower temperature due to its low dispersion caused by weak interaction with the silica support. The weak reduction peak at 694 oC suggests the formation of a small amount of nickel silicate, such as nickel hydrosilicate.37 It should be mentioned that the blank SiO2, Al2O3 and REY supports presented no H2-TPR reduction peaks, consistent with previous studies.47-49 The below TPR data revealed important information with regard to the bulk species of nickel and vanadium present in the FCC catalyst. For the E-cat (Figure 6 I(b)), two prominent reduction peaks are clearly exhibited. The first peak is unequivocally assigned to monomeric vanadium species, whereas the second peak is an alumino/silicate species of the metal contaminants, which could be a combination of these species, AlVO4, NiAl2O4 and nickel

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hydrosilicate, produced by the interaction of metal contaminants with the matrices and zeolite components in FCC catalyst.

(II)

(I)

831

816 737 TCD signal (a.u)

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583 810

(a) (a) (b)

(a1)

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(d) 0

200

400

600

847

800

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500

(b) 1000 0

200

400

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800

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Temperature (oC)

Figure 6. H2-TPR profiles of catalyst samples; I: (a) 2.6 wt% V cat, (b) E-cat-1, (c) E-cat-2, and (d) F-cat; II: (a) 1.2 wt% V cat, (a1) reduced 1.2 wt% V cat, (b) 2.0 wt% Ni cat, and (b1) reduced 2.0 wt% Ni cat.

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818 Ni/Al2O3 460 TCD signal (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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707 795

Ni/SiO2 408

694 594

Ni/REY

756 751

549

V/REY V/Al2O3

525

V/SiO2 200

400

618

600

800

o

Temperature ( C)

Figure 7. H2-TPR profiles of contaminant metals (vanadium and nickel) supported on different components of FCC catalyst. 3.3 Species and Nature of Coke The FTIR spectra of carbons extracted from the coked catalyst samples show similar bands at 730, 1093, 1394, 1628 and 3440 cm-1 (Figure 8(I)), indicating the presence of the same types of carbon species. Specifically, the peaks at ca. 1093 and 1628 cm-1 correspond to ethereal (C-Ostr) carbon species and coke having aromatic skeleton, respectively.23,50 The existence of methyl/methylene groups is verified by the peak at ca. 1394 cm-1, while the 730 and 3440 cm-1 bands are ascribed to C-H out of plane in graphite and surface condensed O-Hstr vibration of hydroxyl functional group, respectively. The presence of different carbon species as shown by the IR spectroscopy results were further collaborated with 13C CP/MAS NMR analysis. As shown in Figure 8(II), aromatic carbon species dominates the carbon layer deposited on the catalyst. Obviously, in both extracted coke and coked catalyst samples, carbon consists of largely hydrocarbon groups that are inherent in the heavy oil used in this study. In order to identify the 20


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crystalline structure of carbon formed on the FCC catalyst, the extracted coke samples were characterized by XRD. The results showed a peak at ca. 2 26.5o, suggesting that the coke may compose of graphitized carbon (Figure 8(III)). However, with only the XRD data, it was difficult to prove if coke was actually graphitic due to the overlapping of the graphite carbon peak of (002) plane with the silica peak at ca. 2 26.7o. Therefore, Raman spectroscopy was employed as a complementary tool. As can be seen from the Raman spectra (Figure 8(IV)), the D and G bands at 1368 and 1594 cm-1, respectively, of the graphite layer are clearly observed. The presence of prominent D bands suggests the defects in the microstructure of the carbon layer, indicating the presence of amorphous structures in the deposited coke.51 7x108

300

(d) (c) (b) (a)

(I) 250

(II) Aromatic C

6x108 5x10

8

4x10

8

3x10

8

2x10

8

Carbonyl C

200

Intensity (a.u)

transmittance (%)

0.3 wt% V-ST 1.0 wt% Ni-ST Fresh cat E-cat-1 1394

(d)

3115

(c) 150

(b) 100

C-O-C; C-OH Aliphatic C Pyrollic C

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8000

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(a) 8000 (b) (c) (d) 6000 (e)

1594 Fresh cat E-cat-1 1365 G-band 1.0 wt% Ni-ST D-band 0.3 wt% V-ST 0.3 wt% V-ST coked cat

offset 1000

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0 10

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Raman shift (cm )

Figure 8. (I) FTIR spectra, (II) 13C CP MAS NMR of fresh coked catalyst and the coke extracted from Ni impregnated catalyst, (III) XRD patterns of coke extracted from spent catalysts; and (IV) Raman spectra of extracted coke samples.

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3.4 Residence of Coke The reduction in pore volume of a catalyst after reaction is indicative of coke deposition in the pores. Table 3 shows the textural properties of the metal poisoned, coked and regenerated catalyst samples. As can be seen, there are large variations in the different physical properties. It is obvious there are large changes in the textural properties between the unsteamed poisoned catalyst and their coked counterparts. However, after regeneration, the catalysts recovered most of their lost properties with minute losses in the surface area and pore volume, possibly due to the occupation of pores by refractory carbon deposits and/or volume degradation caused by heating effect. The relative variations in these catalyst properties reveal that coke occupies the micropores of the unsteamed catalyst, while it settles on the surface of the steamed ones without the blockage of micropores. These results imply that in the real FCC unit coke initially deposits, covers and later blocks the active sites on the zeolite component during cracking, leading to the decay of catalyst activity. During regeneration, the delta coke is burned off, while the stripping coke likely settles on the matrix components, especially for metals poisoned FCC catalyst.52 The results obtained in this study collaborate with those reported by Occelli et al.18 which indicated that coke distributes between the micropores and mesopores of a FCC catalyst. Similarly, Roncolatto et al.20 found that coke on the spent industrial FCC catalyst located majorly on the outer surface, a layer comprising mostly the matrix. As is well-known, for FCC catalyst, the micropore area and micropore volume are assumed to be contributed by the zeolite component, whereas the matrix largely contributes mainly macropores and mesopores.

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Table 3. Textural Properties of Catalyst Samples Samples

Poisoned catalysts Coked catalysts Regenerated Catalysts SABET MPA MEPA TPV MPV ∆SABET ∆MPA ∆MEPA ∆TPV ∆MPV ∆SABET ∆MPA ∆MEPA ∆TPV ∆MPV 232.9 148.5 84.4 0.201 0.077 164.5 141.2 23.3 0.094 0.074 5.6 6.9 (1.4) (0.003) 0.004

F-cat. cal-550a Fresh cat. ST760 a 177.1 104.7 72.4 0.207 0.054 36.9 27.8 9.2 0.018 0.014 2.9 4.4 (1.5) (0.002) 0.002 0.3 wt% V-(N) cal-550 a 232.2 142.7 89.5 0.201 0.074 – – – – – – – – – – 0.3 wt% V-(N) cal-760 a 220.8 148.4 72.4 0.199 0.077 143.8 124.6 19.2 0.089 0.065 8.1 5.8 2.3 0.007 0.003 0.3 wt% V-(N) ST-760 a 75.8 20.0 55.8 0.163 0.010 19.7 6.7 13.0 0.017 0.003 5.5 (1.5) 7 0.003 (0.001) 2.6 wt% V-(V) cal-550 a 217.7 137.9 79.8 0.186 0.071 173.7 131.7 42.0 0.112 0.068 4.6 (4.0) 8.6 0.002 (0.002) 1.0 wt% Ni-(N) cal-550 a 235.9 146.5 89.5 0.198 0.076 – – – – – – – – – – 1.0 wt% Ni-(N) cal-760 a 223.5 142.5 81.0 0.199 0.071 177.5 139.8 37.6 0.116 0.070 4.1 (7.0) 11.1 0.006 (0.006) 1.0 wt% Ni-(N) ST-760 a 152.3 85.2 67.1 0.187 0.044 28.8 18.2 10.6 0.019 0.009 3.6 1.2 2.4 0.002 0.001 3.0 wt% Ni-(Nt) cal-550 a 233.8 136.9 96.9 0.198 0.071 182.1 132.1 50.0 0.116 0.069 15.3 (4.5) 19.7 0.008 (0.002) a o Values in bracket refer to absolute value; cal=calcination, ST=steam treatment, the number refers to treatment temperature ( C); SABET = BET surface area, m2/g; MPA = micropore area, m2/g; MEPA = mesopore area, m2/g; TPV = total pore volume, cm3/g; MPV = micropore volume, cm3/g; ∆ = change with respect to poisoned catalysts, for example, ∆SABET (Coked catalyst) = SABET(Poisoned catalyst)SABET(Coked catalyst)

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Comparing sample 0.3 wt% V(N) ST-760 with sample 1.0 wt% Ni(N) ST-760 (Table 4), it can be seen that these two samples differ by ca. 31% in the relative change in MPV. Therefore, it is reasonable to deduce that more coke prefers to reside in the matrix component of catalyst contaminated with nickel in comparison to that contaminated with vanadium. This phenomenon may be due to the fact that nickel preferentially locates on the surface layer,6-7 while vanadium distributes throughout the interior of the FCC catalyst, as previously confirmed by electron microscopy techniques.9 The surface layer of the FCC catalyst is well-recognized to be composed of the matrix, while the interior has majorly the zeolite particles. To further clarify this hypothesis, by measuring the pore volume degradation of the catalysts after steaming using nitrogen sorption, an estimation of the probable location of nickel and vanadium clearly reveals that these two metals are indeed preferentially located in the matrix and zeolite, respectively (Figure 9). Table 4. Relative Changes in Textural Properties of Coked Catalysts Samples F-cat. cal-550 Fresh cat. ST-760 0.3 wt% V(N) cal-760 0.3 wt% V(N) ST-760 2.6 wt% V(V) cal-550 1.0 wt% Ni(N) cal-760 1.0 wt% Ni(N) ST-760 3.0 wt% Ni(Nt) cal-550

SABET 70.63 26.13 65.13 25.99 79.79 79.42 18.91 77.89

Properties (%) MPA MEPA 95.08 27.61 33.21 16.00 83.96 26.52 33.50 23.30 95.50 52.63 98.11 46.42 21.36 15.80 96.49 51.60

MPV 96.10 32.56 84.42 30.00 95.77 98.59 20.45 97.18

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90

Ni 80 70 60

%∆MEPV

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50 40 30 20

V 10 10

20

30

40

50

60

70

80

90

%∆MPV

Figure 9. Pore volume degradation by vanadium and nickel on steamed catalysts.

Table 5. Coke Content and Behavior for Catalyst Samples Samples F-cat.cal-550 Fresh cat. ST-760 0.3 wt% V(N) cal-760 0.3 wt% V(N) ST-760 2.6 wt% V(V) cal-550 1.0 wt% Ni(N) cal-760 1.0 wt% Ni(N) ST-760 3.0 wt% Ni(Nt) cal-550 a

Coke (wt.%) 6.11 2.87 5.71 2.24 7.23 7.41 3.94 9.51

Coke (g) 0.057 0.027 0.053 0.021 0.068 0.069 0.037 0.089

VR (cm3/g) 0.026 0.012 0.024 0.009 0.030 0.031 0.017 0.040

VA (cm3/g) 0.074 0.014 0.065 0.003 0.068 0.070 0.009 0.069

∅ a 0.346 0.860 0.368 3.131 0.446 0.444 1.836 0.578

∅ = ; VR is the real volume occupied by coke; VA is volume inaccessible to nitrogen caused

by coke deposition

Table 5 shows the coke content and coking behavior of catalyst samples. VR refers to the real volume occupied by coke, given as the mass of coke per gram catalyst divided by the density of coke, which was ideally assumed to be graphite in this study, (density of graphite = 2.23 g/cm3),53 as proven with the Raman spectroscopy (Figure 8(IV)). VA is the change in micropore 25


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volume between poisoned catalyst and its coked counterpart. The ratio (∅), denoted as the index of micropore blockage by coke, enables the determination of an important characteristic and effect of coke. If ∅ ≈ 1, all of the coke is located in the micropores, without micropore blockage; if ∅ ≪ 1, micropores are blocked; and if ∅ ≫ 1, coke is located in both micropores and larger pores.54 For all samples, after steaming, the coke amount significantly decreases and ∅ increases, demonstrating that the steaming treatment collapsed some of the micropores where coke supposedly resides in. In addition, contaminant metals affect the amount of coke formed on the catalyst. For unsteamed samples, a low metal doping (2.6 wt.%) causes a significant increase of coke formation. For steamed samples, a much higher ∅ is obtained on metal doped samples compared with the fresh sample without metal doping. Especially, for sample 0.3 wt.% V-(N) ST-760, ∅ is as high as 3.131, which is 4-fold higher than that of Fresh cat. ST-760. This implies that on steaming, metals may migrate into the interior of catalysts, occupying the micropores. As a result, the acidic sites in the micropores are partially destroyed, thereby decreasing catalytic activity which leads to lower coke production.55 From the above results, it is possible to determine the effects of V and Ni on the location of coke. For the catalysts in this study, in the unsteamed catalysts, coke locates in the entrance of micropores, blocking access to active sites in micropores, in the steamed fresh sample, the deposition of coke is not necessarily responsible for pore blockage, while in the steamed metal doped catalysts, coke is distributed within both micropores and larger pores, majorly in the larger pores owning to the initial blockage of micropores by metals. It should be noted that, in the laboratory deactivated catalysts, metals are first deposited on the catalyst before being evaluated, unlike in commercial FCC unit, where the metals and coke would deposit simultaneously during cracking. Therefore, the results herein 26


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presented may not represent in entirety the actual coking behavior in the presence of contaminant metals in the real FCC unit.

4. CONCLUSIONS The species and locations of FCC catalyst contaminants including vanadium, nickel and coke were investigated. Detailed characterization results indicate the existence of vanadium in +4 and +5 valance states on the surface of industrial spent catalysts. Nickel exists mainly as NiAl2O4 on both the surface and bulk of the catalyst, with some traces of NiO and hydroxylated nickel. The coke on the modelled E-cats, which consists of graphitized carbon layers with dominantly aromatic carbon species, distributes within the micro- and macro-pore spaces. In the unsteamed and metal doped catalysts, coke locates in the entrance of micropores, blocking the access to active sites, while in the metal doped and steamed catalysts, coke distributes within both the micropores and macropores, majorly in the macropores owning to the initial blockage of the micropores by metals. This study therefore affords the understanding of the location and species of the metals poisons, and the corresponding effect on coke deposition during the real FCC unit operation utilizing VGO feedstock and could enable the rational design of FCC catalysts with high metal resistance.

ACKNOWLEDGEMENTS This work was financially supported by the Joint Funds of the National Natural Science Foundation of China and China National Petroleum Corporation (U1362202), Natural Science Foundation of China (51601223, 21206195), the Fundamental Research Funds for the Central Universities (14CX02050A, 14CX02123A), Shandong Provincial Natural Science Foundation

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(ZR2012BM014), and the project sponsored by Scientific Research Foundation for Returned Overseas Chinese Scholar.

References

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Energy & Fuels

For Table of Content use only coke Ni V

NiAl2O4

High Ni concentration

V5+

Matrix

V2O5 AlVO4

(a) 1

(a )

NiAl2O4

High V

(b)

concentration 514

516

518

zeolite200

520

Binding energy (eV)

400

600

800

Temperature (oC)

Surface species 870

1

(b ) NiO

Bulk species 865

860

855

850

Binding energy (eV)

33


Location and Surface Species of Fluid Catalytic Cracking Catalyst

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